The present invention relates to a system for welding and a method for employing the system, and more particularly to a combination of Metal-Inert-Gas (MIG) welding and Plasma Arc welding. MIG welding technology has been widely known for many years and is broadly used in industrial applications.
The MIG process, also known as Gas Metal Arc Welding (GMAW), incorporates automatic feeding of a continuous consumable electrode that is shielded from the atmosphere by an externally supplied gas. Of special importance is the transfer of metal from the consumable electrode to the workpiece being welded. The transfer may occur via any one of the following three basic modes: (a) short-circuiting transfer, (b) globular transfer, and (c) spray transfer.
Operation in the spray transfer mode is highly desirable and possible for example in an argon-rich shielding gas. The spray is a highly directed stream of discrete drops, the drops being accelerated by arc forces sufficiently strong to overcome the effects of gravity. The spray transfer mode is possible if the arc current is higher than a so-called transition current, the value of which depends on wire diameter and type of material. However, the high deposition rate typical to the spray transfer mode may produce a weld pool too large to be supported by surface tension in vertical or overhead positions. Also, both the deposition rate and its rate of increase become larger as the welding current increases. This leads to excessive electrode melting and excessive consumption of filler metal.
The above considerations limit the welding speed and the thickness of the material that may be welded in one pass when operating in any of the three modes, leading to the need for multipass welding with different groove types. In addition, a large welding pool produces a high level of welding distortion. If the rate of heat transfer to the workpiece can be accelerated without increasing the filler metal deposition rate, the penetration depth and welding speed can be dramatically increased.
It is known that a combination of commonly used MIG welding with plasma arc welding (PAW) or with Tungsten Inert Gas (TIG) welding can improve welding fusion and increase productivity. The PAW process produces superior quality welds in two: low-current and high-current (keyhole) arc modes. Its main drawback is low speed. The keyhole mode speed is limited by the physical conditions in the weld puddle. Regular speeds for the keyhole mode range from 10 inch per minute (ipm) to 15 ipm.
U.S. Pat. No. 2,756,311 describes a high-speed tandem arc welding employing two or more inert gas shielded arcs in a tandem arrangement, in which a leading arc (MIG) effects penetration and a subsequent (TIG) arc eliminates undercuts by shaping the welding bead without further penetration. Different types of magnetic “focusing” coils are placed around TIG torches co-axially with the tungsten electrodes. However, these focusing or “magnetic deflecting” coils stabilize only the TIG arc, thus making impossible to control the distance between MIG and TIG arcs at their point of impingement with the workpiece during the welding. This is a major disadvantage that leads to the substantial reduction of the combined process welding speed and penetration depth.
U.S. Pat. No. 3,519,780 proposes an augmentation of MIG by TIG torches, by applying different pulses for the MIG and TIG in some sequence. Two separate torches are used without having any electromagnetic influence between the two resulting arcs. Moreover, the average heat input is very limited, as is the case in any pulsed current approach vs. continuous current applications. The particular sequence of the current applied enables only input from only one electrode at a time, thus dramatically reducing the penetration ability of the augmented arc process.
U.S. Pat. No. 3,549,857 discloses another example of an augmented MIG process where two separate torches, namely MIG and TIG, are arranged in tundem and placed so that there is no common welding pool. The main idea is to provide a separate influence of the two energy sources on the weldment, one for the surface preheating and the other for the actual welding. Both torches are connected to the positive and negative terminals of a common power supply, limiting the ability to optimize the energy input by using independed power sources to feed the MIG and TIG electrodes.
In U.S. Pat. No. 3,612,807, A. J. Lifkens and W. G. Essers present a method and apparatus for plasma welding with axial feeding of filler wire. This idea is further developed in U.S. Pat. Nos. 4,016,397, 4,039,800, 4,220,844, 4,205,215, 4,234,778, and 4,142,090. A plasma arc is maintained between a non-consumable electrode (non axial) and a workpiece. The plasma stream is constricted by a nozzle. A consumable electrode is guided into the plasma stream coaxially therewith, and a second (MIG) electrode arc is maintained between the end of the consumable electrode and workpiece. The end of the consumable electrode and the MIG arc are both immersed in the plasma stream. Both electrodes must have the same polarity. Due to the axial feeding of the consumable electrode, it has a long region of contact with the plasma arc, leading to its preheating. This results in extremely high deposition rates, without actual penetration to the work piece or substantial increase in the welding speed.
U.S. Pat. No. 5,990,446 describes a process in which two (MIG and TIG) torches are placed on opposite sides of a workpiece. Consumable and no-consumable electrodes have an opposite polarity and use a single power supply. One major disdavantage is that the TIG process requires a constant current power supply, while the MIG process requires a constant voltage power supply. In addition, placing torches on opposite sides of a work piece restricts the common electrical and gas dynamic influence of the arcs to the welding pool, thus limiting the penetration ability and welding speed.
U.S. Pat. No. 6,693,252 discloses methods and apparatuses for Plasma-MIG welding or TIG-MIG welding. The methods include a Plasma or TIG torch for following along a weld path a MIG torch or vice-versa. A constant distance is maintained between the torches, and the angle of the torches relative to the workpiece may vary before welding. The MIG process may be performed EP or EN in various embodiments. The suggested approach of having a fixed distance between torches cannot provide a controlable distance between arcs at the point of their impingement on the specimen during the welding cycle. A “controllable” distance depends on the choosen electric current(s), welding speed, weldable materials and joints configuration. It does not depend on the distance between torches or the angle of the torches relative to the workpiece.
As mentioned above, the MIG process is relatively fast but has limited penetration ability, leading to the need for multipass welding with different groove types. Also, relatively high speed MIG welding is limited by the “undercutting” conditions of the final weld, when the fused zone of the work piece is not filled completely with the molten metal.
Plasma welding provides good final weld quality, however, the welding speed is usually restricted to less than 15 ipm. With reference to FIG. 1, it is well known to those skilled in the art that when the plasma arc welding speed is too great, a weld pool 10 created by a plasma arc 12 remains behind with respect to a welding direction 14 at the intersection of the axis of the plasma electrode with the surface of workpiece 16, causing undercuts and lack of penetration.
Prior art is limited in high-speed applications due to the well known effect of the plasma arc falling behind the arc axis during high speed welding. In prior art, the electric current rates passing through the MIG and plasma electrodes must be limited due to the risk of the plasma arc being blown-up by the MIG arc. This in turn limits the penetration ability of the combined process. Finally, in prior art there is no common body for the consumable and non-consumable electrodes, which significantly limits practical applications due to the dimensional constrains.